That ability can be used whenever you have ways to make Diplomacy check to affect a crowd of people, such as when doing a Performance Combat (Introduced in Ultimate Combat):
While combatants do the actual fighting in performance combat, the crowd remains an active participant in these bouts. The audience can bolster or demoralize the competitors with their enthusiasm or scorn for what they see on the battlefield, with serious results. A crowd’s attitude is similar to a nonplayer character’s attitude when a character uses the Diplomacy skill. The DC of performance combat checks to improve crowd reaction is tied to the crowd’s starting or current attitude. Each time the combatants do something spectacular on the battlefield, they have the opportunity to parlay that success into a better crowd reaction, but missteps can also create contempt among the crowd.
(...)
Crowds can be bribed or coerced. Typically a successful bribe or attempt at coercion involves paying some amount of gold based on the crowd size, and succeeding at a Diplomacy check with a DC based on the size of the crowd. If the gold is paid and the Diplomacy check fails, the side gains a penalty for cheating instead, though the gold is still spent. If the check succeeds, the DC to improve crowd reaction and gain victory points decreases by 2. For every 5 points over the DC by which the combatants make the check, the DC to improve the crowd’s attitude or to gain a victory point decreases by 1.
Or even on Verbal Duels (introduced on Ultimate Intrigue):
An incidental crowd, such as a crowd at a market, generally pays attention only to particularly impressive speakers or performers. A successful DC 20 Bluff, Diplomacy, Intimidate, or Perform check and the appropriate political arguments attract a small crowd of up to a dozen observers. Getting the attention of a larger group generally requires a result of 25 or 30, at the GM’s discretion. If an audience the PCs wish to incite to riot is watching someone else, an opposed Diplomacy, Intimidate, or Perform check is required against the current speaker, whether it be a performer, bazaar vendors, or a politician.
Do note that though Ultimate Intrigue was released much later (2016), Ultimate Combat (mid-2011) has a closer release date to the Advanced Players Guide (late-2010), being separated only by a few months.
On the other hand, Intimidate checks were already possible against a group of people by using Dazzling Display (from the Core Rulebook).
Also, nothing in the skills listed actually prevent them from being used on more than one creature at once, other than a few specific uses (like feinting or palming), it merely describes the effects of using those skills against another creature. Hiding an object or disguising yourself, for instance, will affect everybody in a crowd. So it is one of those cases where GM Fiat is required, and you will see table variation on the subject.
Personally, I think I have two or three times asked a player to roll Intimidate or Diplomacy against a group of NPCs, which made perfect sense in the context. In one, the player had just one-hit KO'ed the "boss" of a group of thugs and asked me if he could roll an Intimidate check, he passed and it affected all thugs, who ran away. The other that comes to mind was when (this same player, another campaign) tried to convince a caravan from leaving their camp and returning back to town because a deadly storm was coming their way, which I asked for a single Diplomacy check against the elders of that group.
